This invention relates to axial flow fans, for example, fans designed for use in automobiles or heating, ventilation and air conditioning (HVAC) systems.
Axial flow fans for producing flow are employed in a wide variety of applications. For example, a fan is placed within the engine compartment of an automobile to move air through a radiator. When the automobile is operating, particularly at low speeds or idling, noise generated by the fan can be heard by the occupants within as well as by those outside the automobile. In this application it is desirable to provide a fan that is strong (sufficient airflow), compact (due to space limitations) and relatively quiet.
One type of axial flow fan includes a number of blades, each having a root end secured to a hub that is driven by a rotating shaft of an electric motor and from which the blades extend radially outward. The blades may be of any suitable design, but are generally "pitched" at an angle to the plane of the fan rotation to generate an axial air flow as the blades rotate.
The electric motor is secured within a cylindrical band known as a motor ring with the rotating shaft coaxial with the ring. A number of vane members or stators extend from the motor ring to a second cylindrical housing or shroud to provide structural support to the electrical motor during operation.
The interaction of the rotating blades with the fixed stators generates noise in the audible range of the human ear called blade rate tones. Blade rate tones and their harmonics are generated when a blade passes over a stator at any given time. The blade rate tone has a frequency proportional to the product of the number of blades and the rotational velocity of the fan. When more than one blade passes over a stator at the same time, the blade rate tones are said to "stackup" causing their magnitude to increase.